Pierce was in so much pain from a right knee infection that the All-Star forward called team trainer Ed Lacerte on Wednesday around 3 a.m, coach Doc Rivers said. Lacerte went to Pierce’s house and took him to the hospital, where they drained fluid from Pierce’s knee.

Pierce didn’t make the flight to Orlando for the Christmas Day game against the Magic. He is expected to miss at least two weeks.

“What he’s going through is pretty significant,” Celtics guard Ray Allen said. “It is serious. Because me knowing Paul, he’s always practicing and he’s always playing, so for him not to be here, it must be very significant.”

The injury surprised the Celtics.

The 2008 NBA finals MVP had played in every game this season, leading the team with an average of 18.2 points. Pierce complained of soreness in his knee after Tuesday night’s game, when he missed his first 10 shots against the Indiana Pacers but wound up scoring 21 points in the win.

The team left the arena thinking he was fine.

“It’s amazing, he won the game for us the night before and he woke up in the middle of the night hurting,” Rivers said.

The Celtics are on a six-day, four-game road trip that will also take them to the Clippers, Golden State Warriors and Phoenix Suns before returning home for a Jan. 2 game against Toronto. Rivers said it was doubtful Pierce would join them on the trip.

The good news for Boston: Kevin Garnett said he will play against Orlando after missing the last game with a thigh bruise.

Garnett was out with knee injuries in last season’s Estern Conference semifinals against the Magic, who eliminated the then-defending NBA champions in seven games. Glen “Big Baby” Davis also practiced, but Rivers said he’s unlikely to play Friday because the forward is still recovering from a broken right hand.

But getting Garnett back should give the Celtics a boost.

“I’m playing,” Garnett said. “That’s all that matters. Details are not important.”

Even Garnett’s attention was focused on his teammate.

Players have been sending Pierce text messages and words of encouragement from Orlando, where the Celtics arrived Wednesday along with family members for the holidays. They came early to enjoy the area’s theme parks and attractions with their families – Rivers makes his permanent home in central Florida – but each said that Pierce was on his mind.

“It’s unfortunate for Paul’s situation because it was kind of like a freak accident, or a freak situation,” Garnett said. “It’s really weird when you get the details of it.”